swiftlang/swift

[SR-5337] Improve diagnostic for 'var' in parameter position

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#47,911 opened on Jun 29, 2017

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compilerdiagnostics qualitygood first issueimprovement

Description
Previous ID SR-5337
Radar None
Original Reporter @CodaFi
Type Improvement
Votes 1
Component/s Compiler
Labels Improvement, DiagnosticsQoI, StarterBug
Assignee None
Priority Medium

md5: df452a4dc52959caff3a154681f76513

Issue Description:

For code that looks like this

func foo(inout x : Int) {}

or

func foo(_ f : (Int) -> Void) {}

foo { (var x) in
  
}

We used to offer to remove the errant 'var' specifier and extract it into a variable in the body of the function/closure. This diagnostic necessitated a compromise in our modeling of parameter declarations that I have removed. In its place, it would be nice to have a diagnostic of similar quality.

Some ideas would be to redo the old diagnostic, but by first "enqueueing" it then emitting the variable into the body of the function/closure after we parse the opening brace/'in' after a parameter list. Or we could simply do what we do for 'inout' and remove the 'var' specifier then assume the user actually wanted 'inout', etc.

Contributor guide

Tech stack
swiftcpp
Domain
developer experiencetooling
Issue type
feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
over 1 week
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Swift programmingCompiler internalsDiagnostic engineering
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
25
Research direction
First, study the Swift compiler's parsing of parameter declarations and the current diagnostic handling for 'var' in that position. Look at the git history for recent changes to the diagnostic system that removed the old compromise. Then, design a new diagnostic that removes the 'var' specifier and introduces a variable declaration in the function/closure body. The implementation should likely modify the parser or the diagnostic emitter to enqueue a fix it after parsing the opening brace. Review related files in the 'lib/Parse' and 'lib/Sema' directories.